Method of designing dynamometers



Oct. 13, 1942. A. R. ROGOWSKI 2,298,725

METHOD OF DESIGNING DYNAMOMETERS Filed June 6, 1941 4 Sheets-Sheet 1 1 0 if? I $3 0 fluyus'l'us H. Rayowslii BY REJ'IJTHNCE Oct. 1942- A. R. ROGOWSKI METHOD OF DESIGNING DYNAMOMETERS Filed June 6, 1941 4 Sheets-Sheet 2 Ev QNV QQv MS M INVENTOR fluyus'lus R.Royowslii Oct. 13, 1942. A. R. ROGOWSKI METHOD OF DESIGNING DYNAMOMETERS Filed June 6, 1941 4 Sheets-Sheet 3 THOUSANDS OF RPM INVENTOR qus'l'us R.Royowsiii All Patented Oct. 13, 1942 UNITED STATES PATENT OFFICE METHOD OF DESIGNING DYNAMOMETERS Augustus R. Rogowski, Needham, Mass.

-Application June 6, 1941, Serial No. 396,828

Claims.

This invention relates to electromagnetic power absorption devices using the eddy current principle. The invention may be used specifically in connection with electromagnetic dynamometers and brakes and other devices of similar character.

The invention is primarily concerned with a method of providing, in an eddy current power absorption device, a torque curve of a shape suitable for meeting the condition of use to which the machine will be put. The invention also includes the construction of the machine incorporating parts designed according to the method of the invention.

Previous electromagnetic dynamcmeters and power absorption devices using the eddy current principle have often suffered from speed instability when used with prime movers having relatively constant torque-speed characteristics, usually due to the fact that the absorption torque of the dynamometer did not cross the output torque of the prime mover at such an angle that increasing R. P. M. resulted in a dynamometer torque greater than the prime mover torque. This disparity often occurred over a large portion of the operating speed range, necessitating the action of complicated and expensive auxiliary equipment such as exciters, electronic field controls, and the like, if anything like suitable results were to be obtained.

By the present invention, it has been found that in the simple type of dynamometer or eddy brake comprising a disk of nonmagnetic conducting material rotating between one or more pairs of magnetic poles, the rotative speed at which the maximum torque occurs may be shifted to higher or lower speeds, as desired, by substituting materials of higher or lower electrical resistivity.

Accordingly, the invention therefore relates to the method of determining the construction of an electromagnetic dynamometer or brake to meet the torque requirements of a prime mover with which it is to be used. Further, the invention contemplates the particular design of the device when made in accordance with the method of the invention.

An additional feature of the invention resides in the provision of a standardized dynamometer structure, in which the rotor may be changed at will from one material to another whereby the revolutions per minute at which the maximum torque is produced may be varied to meet the requirement of varying rime movers with which it is to be used.

These and other objects of the invention will become more apparent as the description proceeds with the aid of the accompanying drawings, in which:

Fig. 1 is a side elevation of an electromagnetic dynamometer incorporating the invention.

Fig. 2 is an end elevation of the machine shown in Fig. 1 looking from the right.

Fig. 3 is a section on the line 33 of Fig. 1.

Fig. 4 is a side elevational detail of the housing locking mechanism.

Fig. 5 shows some of the torque curves that may be produced as a result of the invention.

Fig. 6 shows the nature of the eddy currents at low speed.

Fig. 7 shows the displacement of the eddy currents at high speed.

Figs. 8 and 9 show the low and high speed vector diagrams.

Fig. 10 illustrates the manner in which the torque curve of a dynamometer using a rotor of selected material may be varied by altering the number of magnets, the strength of the field current, or both.

Referring now to Figs. 1 to 4, a preferred form of electromagnetic dynamometer using the invention is shown. This consists of a base 2 having upwardly extending spaced supports 4 and 6 which carry a housing 8 within which is mounted a rotor [0 carried by a shaft [2 which may be connected in any suitable manner to a prime mover.

The details of this construction are as follows.

Referring to Fig. 3, which is a section on the line 3-3 of Fig. 1, the supports 4 and 6 have mounted within bearing races l4 and I8 respectively. The housing 8 has centrally extending flanges 22 and 24 which carry bearing races 26 and 30 respectively, corresponding to the support races l4 and I8. Between these races are mounted a plurality of ball bearings 34 and 36. From this construction, it is clear that the housing 8 is supported for rotative movement with respect to supports 4 and 6.

Rotation of the housing with respect to the supports, however, is limited by means of a locking device 38, shown in Figs. 1, 2, and 4. This locking device may take any desired form, but in the construction shown, the housing 8 has a suitable recess 40 in its side into which extends a key 42, generally elliptical in shape, as shown in Fig. 1. Key 42 is mounted in support 6 for limited rotative movement under the control of a lever 44, connected link 46 and lever 48. Movement of lever 68 is in turn controlled by a piston 50 operated in any suitable manner.

As can be seen from Fig. 1, movement of lever 48 to the right will result in rotating key 42 to the left approximately 90. By adjusting the dimensions of key 62 with respect to recess 46, movement of housing 8 through a limited angle will be permitted when the key is in vertical position, while movement of housing 8 will be eliminated or greatly reduced when the key is in horizontal position. The purpose of the locking device will become more apparent hereafter.

Flanges 22 and 24, that carry the inner ball races 26 and 36, are continued inwardly to form the hubs 52 and 54 which carry outer ball races 56 and 56, while mounted in a corresponding position on shaft I2 are ball races 66 and 62. Between these two sets of ball races are a plurality of ball bearings 64 and 66. Housing 8 is in turn carried by hubs 52 and 54.

At the center of shaft I2 and fixed thereto are a pair of disks 68 and I6 adapted to secure therebetween a larger disk I2, all of Which together constitutes the rotor.

From the description thus far, it will be seen that shaft I2, carryingrotor 12, may revolve within the confines of housing 8, and housing 8 in turn is free to move a limited amount in supports 4 and 6, depending upon the setting of key 2.

Housing 8 may be formed in any desired manner, but in the present construction comprises two halves I4 and I6 with circumferentially extending flanges l8 and 36 which may be bolted together as shown in Fig. 2. The housing is closed all the way about its circumference except for a limited distance at the bottom between the points numbered 82 and 84 (see Fig. 1). This construction permits the introduction of cooling Water on the rotor through jets or other suitable means and at the same time causes retention thereof in the space within the housing at the outer circumference of the rotor. The water may drain from the housing at the bottom into a sump 86, from which it may be drawn, cooled and recirculated or permitted to drain away as Waste.

Housing 8 has positioned within and spaced radially thereabout a plurality of electromagnetic poles formed by a series of cores, two pairs of which are shown in section in Fig. 3 as 88 and 90 and 92 and 94. Altogether there are four pairs of cores, as may be seen in Fig. 1, the other two pair being numbered 96 and 93.

About each of these cores is wound a coil, four of which are shown in Fig. 3 in section at I66, I62, I64 and I66. There are corresponding coils Wound about the other two pair of cores 96 and 98. The inner ends of the several cores are located close to the rotor I2, near the rim thereof.

Each pair of cores is connected by magnets I68, III], H2, and H4, so that when the several coils are excited by the application of current thereto, a series of magnetic fields will be set up through which the rotor I2 may be moved.

In order to prevent the rotor and cores from becoming unduly heated during operation, cooling means has been provided. All of the cores are hollow, as illustrated at IE6, IIB, I26, and I22, in Fig. 3, so that water may be introduced therethrough to impinge directly upon the rotor.

Water is supplied to the hollow cores by any convenient source of water under pressure which may be lead in through pipe I24 to pipe I26, thence to pipes I28 and I36, and finally to pipes I32, I34, I36, I38, feeding the cores onone side,

and to a corresponding set of'pipes feeding the cores on the other side.

Water flowing through the core passages impinges against the opposite surfaces of the rotor rim, to be flung outwardly under centrifugal force as the rotor revolves, from whence it will drain down into sump 86. The volume and the temperature of the cooling water may be adjusted to keep the rotor at any desired temperature.

On one side of the housing 8 is a short lug I60, having an insert I22 adapted to receive a knife edge connected to any suitable scale, which may measure the rotative effect applied to housing 8 through the rotation of rotor I2. Thus, in Fig. 1, if the rotor be rotated counterclockwise through the magnetic fields of the several electromagnets, the housing will tend to turn to the left. If key 62 has been swung to horizontal position so that the housing is unlocked, permitting limited movement of housing 8, the rotative force may then be measured by the scale restraining lug MB.

A further aspect of the invention resides in the methods of determining the particular material of which rotor I2 may be made to produce torque curves of predetermined characteristics. Obviously, the torque curve produced by a rotor ofgiven material will vary if the physical dimensions are changed or if the magnetic fields are varied. However, if a dynamometer of fixed dimensions and a magnetic field of fixed strength are assumed, then from the method of the present invention it is possible to determine the material of which the rotor may be made to produce a torque curve of desired characteristics. Thus, the rotor, of fixed dimensions, in this case 18 inches in diameter, operating between 1% inch pole pieces, produces torque curves of varying characteristics, as shown in Fig. 5, determined by the material of which the rotor is made. From the following table, the relationship of the electrical resistivity of illustrative materials to revolutions per minute to produce maximum torque may be seen.

E1 t 1 R. P. M; ec ,rica for maxi- Matenal resistivity mum torque Oopper 1.7 185 Duralumm 5. 8 630 Bronze 12 l, 300 Mon 42 4, 560 Stainless steel 69 7, 500 Nichrome 100 10, 900

rial having a resistivity of approximately 55. It

will be observed from Fig. 5 that all of the curves follow a substantially uniformly changing contour, so that power output at revolution speeds less than maximum are likewise proportional to the resistivity of the material used. A substantially straight line curve has been found to exist between the electrical resistivity of the metal used and the revolutions per minute at which maxie mum torque is produced, it the other factors of the dynamometer remain unvaried.

As a result of this invention, it is now possible to produce an electromagnetic dynamometer or brake which will provide a torque curve which reaches a maximum at any desired number of revolutions per minute through the variation of the rotor material without otherwise changing the dynamometer construction.

For example, if it is desired to measure the power output of an engine through its Various speed ranges, a rotor may be selected which will provide maximum torque at some R. P. M. above the speed range under consideration, or at such an R. P. M. that the dynamometer torque curve has a steeper slope than that of the engine, at all points within the required speed range.

In many cases, the maximum torque that may be absorbed by any given dynamometer will not sufiiciently approximate the maximum torque that may be produced by the prime mover to be tested, with the result that such dynamometer cannot be used.

The invention therefore also includes the method of adjusting a dynamometer in such manner that the torque absorbing capacity may be increased or decreased while the R. P. M. at maximum torque absorbing capacity remains constant. This phase of the invention is illustrated in Fig. 10. This diagram is illustrative of a dynamometer of fixed dimensions using a rotor of selected material. The four torque curves, numbered I56}, I52, I54 and I56, all reach their maximum torque at substantially the same number of R. P. M. The torque curve I50 is produced through the utilization of a large number of magnets and a high field current. By reducing the number of magnets, but still using a high field current, a curve such as I52 will be produced. If, on the other hand, the number of magnets remained the same, as in the case of curve I50, and a low field current were used therewith, then a curve such as I54 would be produced.

Finally, if the number of magnets used is reduced and a low field current used therewith, a curve such as I56 will result. However, in all instances it will be noted that the dynamometer will reach maximum torque at approximately the same number of R. P. M.

Superimposed on curves I50, I52, I54, and I56 are typical torque curves of engines to be tested, numbered I58 and I60. These curves intersect with the torque curves to form the angles A, B, C, D, E and F. In each case it will be noted that the angle so formed is positive. That is to say, the torque curve of the dynamometer will cross that of the engine to be tested, thus keeping the latter under control throughout the required speed range.

From Fig. it is believed apparent that suitable adjustments to the number of magnets and field current may be made to any dynamometer using a rotor of selected material, so that a resulting torque curve may be produced which will be adequate to match the power output characteristics of the prime mover to be tested.

Accordingly, the power absorption capacity may be adjusted by varying the number or strength of the magnetic fields acting on the outer area of the rotor, while the R. P. M. at which maximum torque appears may be controlled in accordance with the electrical resistivity of the rotor material.

To change the torque absorbing capacity of the LII dynamometer without varying the R. P. M. at which maximum torque is produced, the electromagnetic field strength may be varied in accordance with the ratios of the square roots of the present torque and the torque desired.

The power absorption capacity and the speeds at which corresponding maximum torque is found may, of course, be otherwise varied by changing the dimensions of the machine and varying the field strength of the electromagnets so that the dynamometer may be additionally adjusted or altered to meet the conditions of practically any prime mover as to its power output and maxi mum speed.

The materials listed are illustrative only. Obviously it is possible to prepare alloys or to assemble rotors of different materials which will have resistivities corresponding to any desired figure,

In some instances it might be found desirable to make the rotor of magnetic, rather than nonmagnetic, materials, in which case the R. P. M. for maximum torque will vary directly with resistivity and inversely with permeability. However, when using magnetic materials, an exact prediction is difficult because permeability varies with the flux density and flux leakage varies in turn with the permeability. In the case of the use of non-magnetic materials, as heretofore described, these conditions are not present and more exact predictions are therefore possible.

I claim:

1. The method of selecting material for a rotor for use in an electromagnetic dynamometer which will result in the development of maximum torque at a predetermined number of revolutions per minute, comprising the steps of measuring the torque of a given dynamometer while said dynamometer is utilizing a rotor of known dimensions and known electrical resistivity and is rotating in magnetic fields of known strength, noting the revolutions per minute of said known rotor at which the torque of said dynamometer reaches a maximum, and then selecting for the desired rotor a material whose electrical resistivity is in substantially the same proportion to the electrical resistivity of the rotor tested as the revolutions per minute of the desired rotor at which maximum torque is required is to the revolutions per minute of the rotor tested at which maximum torque was produced.

2. The method of determining the material of which a desired rotor in a disk type eddy current dynamometer should be made to give maximum torque at a predetermined number of revolutions per minute, where the physical dimensions of the dynamometer are fixed and the magnetic fields are constant, comprising the steps of making a rotor of any selected nonmagnetic conducting material and of known electrical resistivity, testing said rotor in said dynamometer to determine the number of revolutions per minute at which said rotor provides the greatest torque, and then selecting as material for said desired rotor which is to give its maximum torque at said predetermined number of revolutions per minute, a nonmagnetic conducting material having electrical resistivity which bears substantially the same relation to the electrical resistivity of the rotor tested as the revolutions per minute at maximum torque of the desired rotor bears to the revolutions per minute at maximum torque of the rotor tested.

3. The method of determining the material of which a desired rotor in a dynamometer should be made to give maximum torque at a prede termined number of revolutions per minute, where the physical dimensions of the dynamometer, including the rotor, are fixed and the magnetic field is constant, comprising the steps of making a first rotor of any selected metal of known electrical resistivity, testing said first rotor in said dynamometer to determine the number of revolutions per minute at which said first rotor provides the greatest torque, and then selecting, as material for said desired rotor which is to give its maximum torque at said predetermined number of revolutions per minute, a nonmagnetic metal having an electrical resistivity bearing substantially the same relation to the electrical resistivity of said tested rotor as the revolutions per minute at maximum torque of the desired rotor bears to the revolutions per minute at maximum torque of the rotor tested.

4. The method of testing any one of a plurality of prime movers, each designed to produce maximum torque at a different number of revolutions per minute, by means of a single electromagnetic dynamometer without changing the dimensions or the magnetic field of said dynamometer, which comprises ascertaining the electrical resistivity of a rotor of a given material capable of producing maximum torque at a predetermined number of revolutions per minute and then constructing a new rotor of a material having an electrical resistivity bearing substantially the same proportion to the revolutions per minute at maximum torque of the motor to be tested as the said ascertained electrical resistivity bears to the said predetermined revolutions per minute, and then utilizing said new rotor in said dynamometer.

5. The method of designing an electromagnetic dynamometer including the rotor therefor capable of absorbing a predetermined maximum torque at a predetermined number of revolutions per minute, comprising. the steps of making a first rotor of any selected nonmagnetic conducting material and of known electrical resistivity, testing said rotor while using electromagnetic fields of known strength, determining the number of revolutions per minute at which said first rotor provides maximum torque, then selecting as material for said desired rotor a nonmagnetic conducting material having electrical resistivity which bears substantially the same relation to the electrical resistivity of the first rotor tested as the predetermined revolutions per minute at maximum torque of the desired new rotor bears to the revolutions per minute at maximum torque of the rotor first tested, then testing said new rotor while using electromagnetic fields of the same strength to determine the maximum power absorption capacity at said predetermined number of revolutions per minute and finally altering the electromagnetic field strength in the same proportion that the square root of the torque produced by the rotor just tested bears to the square root of the said predetermined maximum torque to be absorbed.

' AUGUSTUS R. ROGOWSKI. 

